The present invention relates to radio receivers and, more particularly, to receivers for double sideband, amplitude modulated signals, such receivers having extremely high frequency selectivity.
The vast majority of radio transmission in the world today employs an amplitude modulated signal comprising a carrier plus upper and lower sidebands containing the transmitted information. The frequency separating the sidebands and the carrier is normally not greatly restricted at the transmitter. However, amplification at the receiver is usually restricted to frequencies less than 2500 hertz from the carrier in order to permit station assignments at the closest practicable frequency intervals. In the United States, for example, assigned broadcast frequencies are normally spaced by 10 kHz, and in Europe by 9 kHz. Another reason for restricting bandwidth at the receiver is the limited signal-to-noise ratio that can be realized with amplitude modulation. Restricting the bandwidth of the receiver to 5 kHz reduces the noise content without greatly comprising the entertainment quality of music or the intelligibility of speech.
The basic receiver type used throughout the world is the superheterodyne. This circuit changes the received frequency to a fixed frequency at which the bulk of the amplification and selectivity is provided. This fixed frequency or intermediate frequency amplifier, as it is called, must pass the carrier and the sideband information to plus-or-minus 2.5 kHz, but provide sufficient attenuation to reject a carrier and its sideband information on an adjacent channel assignment. One index to describe the effectiveness of an I.F. amplifier from a selectivity standpoint is the "shape factor". This is obtained by exploring the shape of the I.F. response with a variable, unmodulated signal. The ratio of the bandwidth at 60db attenuation to the bandwidth at 6db attenuation is the shape factor. Consumer type radio receivers normally have shape factors considerably in excess of 10 while more sophisticated types with a large number of tuned circuits may have a shape factor as low as 3. This may be reduced to 1.5 or even less by the use of complicated mechanical or crystal filter structures. The ultimate, to this point in time, has been the "ideal" selectively characteristic wherein bandwidth remains constant at all levels of attenuation, i.e., the shape factor is 1.
It is one object of this invention, by the use of circuitry to be described, to provide an AM radio receiver capable of having an effective shape factor of less than unity, i.e., wherein the effective bandwidth at 60db signal attenuation is substantially less than the bandwidth at 6db attenuation.
The present limitations in the spacing of assigned broadcast frequencies, imposed by the selectivity levels of the more common receivers, restricts the number of requests for new transmitter assignments which may be granted by government communications commissions. Since the level of new requests for transmitter assignments continues unabated, a practical means to increase station density is highly desirable. Single sideband broadcast has been considered, but is generally regarded as impractical because of the added complexity in receivers, among other reasons. Furthermore, even with present channel spacings, it is sometimes desirable to tune to a signal spaced only 1 or two kHz from an undesired signal. This may occur, for example, when receiving European or Latin American stations in the U.S., and vice versa. This is impractical with conventional receivers since significant selectivity does not occur closer than 2.5 kHz from the desired frequency due to the necessity of passing the sidebands in the usual manner.
A further object of the invention is to provide a receiving circuit for double sideband, amplitude modulated broadcast signals which permits channel spacings a fraction of that now possible without objectionable interference from adjacent carriers.
In the last few years, digital electronics has made great progress and there is considerable interest in providing radio receivers with digital readout, even those intended for consumer use. For these to be generally accepted, however, it is necessary that the displayed frequency accurately indicate the frequency of the station being received. This is not the case with presently available consumer type receivers where considerable ambiguity results. This is caused by the wide I.F. response necessary to pass the sidebands and is compounded on strong signals by the presence of automatic volume control (AVC) which makes it possible to tune over many kilohertz without a noticeable change in signal strength.
It is a further object of this invention to utilize the built-in redundancy of the double sideband signal to provide such a distinct peaking of the tuning procedure that a digital readout system will accurately identify the frequency of the signal being received.
Still another object is to provide a novel and improved receiver for stereo radio signals.